Garmin's Smart Glide and Prop Position

[Will.iam]

No need i have all the proof i need with actual real life experience when practicing simulated engine out glide distances both with the prop knob in and with it out. Its almost as dramatic a change as lowering the gear. Definitely way more effective than deploying the speed brakes. 

[jetdriven]

OK. The stock airplane is 2.2 nautical miles per 1000, what's your number?

 
There is no doubt it's better. I just don't think it's on the order of a third. I'm guessing 5%. 

[PT20J]

When you initially pull the prop back, you feel a big change due to the extra thrust from the stored energy in the prop as @jetdrivenmentioned. It’s the same idea (on a smaller scale) as pulling collective to stop the descent at the end of a helicopter autorotation. The old hand seaplane pilots taught me this when you are on a takeoff slide and have to get off the water NOW for some reason (maybe a half submerged log in your path that you didn’t see). You pull the prop back at full power, pop off the water and push the prop back up. It’s not great for the engine, so it’s an emergency procedure. 

Back to our failed engine scenario, after the initial surge, there is still a positive effect on glide ratio since the prop is turning the engine against friction and compression slower and thus requiring less power which equates to lower drag. The glide ratio is numerically equal to lift divided by drag, so lower drag gives an increased glide ratio. It’s not an enormous difference, but in an emergency it’s good to get all you can get out of the airplane.

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[aviatoreb]

5 hours ago, toto said:

The Garmin marketing stuff suggests that it selects a route based on terrain, fwiw:

https://discover.garmin.com/en-US/autonomi/#sg

 

It would be good to know.

e

[PT20J]

2 hours ago, aviatoreb said:

It would be good to know.

e

[MikeOH]

I'm still skeptical that it is taking the terrain into account for ROUTING.  Sure it takes terrain into account for RANGE as stated. But, the statement, "creates a Direct-to route" gives me pause for adequate terrain CLEARANCE in an IMC glide.  I don't think it routes around terrain that is BELOW you when you start the glide but WILL NOT be by the time you get to that intervening terrain!

[PT20J]

It seems pretty clear. Smart Glide will only select an airport within the glide range ring. It flys a direct route to that airport. Since the glide range ring is calculated using the terrain/obstacle database, it won’t include an airport blocked by higher terrain. So it won’t fly you into a mountain. 

But, it might miss an airport if there is a hill in the way that could be easily circumnavigated, or if there is one of those 2,000’ TV towers common in the midwest between the airplane and an airport.

 

[FlyingScot]

Forgive me for getting sidetracked a bit, but this conversation begs a stupid question I’ve always had — why don’t all high performance single engine aircraft have a full feathering prop - multis do to reduce drag on an engine out, so why not singles?

[1980Mooney]

47 minutes ago, FlyingScot said:

Forgive me for getting sidetracked a bit, but this conversation begs a stupid question I’ve always had — why don’t all high performance single engine aircraft have a full feathering prop - multis do to reduce drag on an engine out, so why not singles?

Cost and weight that far forward of the datum line.  Rocket Engineering installed a Hartzell Full Feathering PHC-C3YF-2UF/ FC7382 Scimitar 3 Blade prop as a part of the M20J Missile conversion to an IO-550A. 

It has an accumulator on the nose of the prop which adds weight.  See 3rd pic of similar PHC-C3YF-2UF Hartzell 3 Blade Propeller W/ Logs (TT: 768.9) (baspartsales.com)

And the cost is incredible.  Ottosen Propeller has one for sale for $31,200.  I don't think that includes the 2 Piece Spinner which is about $4,300 at Ottesen.

C3F00250 - HARTZELL PROPELLER - PHC-C3YF-2UF/FC7382/SM6 - Ottosen Propeller

D-3273-2P - HARTZELL SPINNER ASSEMBLY (POLISHED) - Ottosen Propeller

[FlyingScot]

1 hour ago, 1980Mooney said:

Cost and weight that far forward of the datum line.  Rocket Engineering installed a Hartzell Full Feathering PHC-C3YF-2UF/ FC7382 Scimitar 3 Blade prop as a part of the M20J Missile conversion to an …

A great and very complete answer - thank you. Makes sense. 

[aviatoreb]

6 hours ago, PT20J said:

It seems pretty clear. Smart Glide will only select an airport within the glide range ring. It flys a direct route to that airport. Since the glide range ring is calculated using the terrain/obstacle database, it won’t include an airport blocked by higher terrain. So it won’t fly you into a mountain. 

But, it might miss an airport if there is a hill in the way that could be easily circumnavigated, or if there is one of those 2,000’ TV towers common in the midwest between the airplane and an airport.

 

I agree it SHOULD!

I am not convinced by the wording that it does.

[midlifeflyer]

 

3 hours ago, 1980Mooney said:

Cost and weight that far forward of the datum line. 

I'd add one thing to that. The benefit side of the cost/benefit ratio is  is far more significant in a twin.

[PT20J]

1 hour ago, aviatoreb said:

I agree it SHOULD!

I am not convinced by the wording that it does.

OK, how about this from the GTN Xi Pilot’s Guide.

 

[rpcc]

I have seen this a few times before but the discussion seems to degrade into xyz and there is never a conclusion.

So... does the prop pitch control work if the engine has failed or has been turned off?   If it doesn't work then this seems like a moot point.

[donkaye]

25 minutes ago, PT20J said:

OK, how about this from the GTN Xi Pilot’s Guide.

 

Better yet:

 

[aviatoreb]

28 minutes ago, PT20J said:

OK, how about this from the GTN Xi Pilot’s Guide.

 

Maybe....  a trusting person would say yes - that's it.  A suspicious person would say, did the engineers really correctly consider line of sight occlusions due to a mountain between me and where I want to be?  Surely they did?  But I would want to go out and test it before I would trust it in an actual emergency.  The adjustable altitude ring is not a circle in large part because of varying altitude of the ground.  But I cannot remember ever seeing a large cut out in my range circle due to a mountain in the way - but then again I dont think Ive ever checked explicitly and mostly I fly high. ...something to do soon.

[donkaye]

So, based on the above, the best glide airport provides terrain and obstacle clearance.  It also considers wind and METAR data if a compatible weather source is provided (see 3 above).

[1980Mooney]

1 hour ago, rpcc said:

I have seen this a few times before but the discussion seems to degrade into xyz and there is never a conclusion.

So... does the prop pitch control work if the engine has failed or has been turned off?   If it doesn't work then this seems like a moot point.

Which point is moot?  The prop could be stationary, it could be windmilling, it could be pulled as far back as possible with least resistance, it could be pushed all the way forward (flatest pitch) with maximum resistance or pitch could be stuck where you had it during cruise which might be somewhere in between.  I don't know the inputs for the Smart Glide algorithms but when programming, if you input the "best glide speeds varying by GW" for worst case - a windmilling prop with prop most flat (all the way in) - then it will calculate and guide to (if you have a GFC500) reachable airports under the worst conditions. If your prop is stationary and pulled back with least resistance, then that is all gravy . You will have more altitude (energy) to work with when you are 2 nm. from the airport and have to turn off the autopilot.

Edited March 19, 2023 by 1980Mooney

[midlifeflyer]

27 minutes ago, 1980Mooney said:

Which point is moot?  The prop could be stationary, it could be windmilling, it could be pulled as far back as possible with least resistance, it could be pushed all the way forward (flatest pitch) with maximum resistance or pitch could be stuck where you had it during cruise which might be somewhere in between.  I don't know the inputs for the Smart Glide algorithms but when programming, if you input the "best glide speeds varying by GW" for worst case - a windmilling prop with prop most flat (all the way in) - then it will calculate and guide to (if you have a GFC500) reachable airports under the worst conditions. If your prop is stationary and pulled back with least resistance, then that is all gravy . You will have more altitude (energy) to work with when you are 2 nm. from the airport and have to turn off the autopilot.

The mootness can also depend on where the failure occurs. Altitude, terrain, and airport density may be an bigger factor for success than the (significant) 450 feet of altitude  @donkaye found with a coarser prop pitch. 

[ArtVandelay]

I would think it would use the static default setting but be able to calculate the glide ratio and adjust as needed.

[PT20J]

10 minutes ago, ArtVandelay said:

I would think it would use the static default setting but be able to calculate the glide ratio and adjust as needed.

According to Garmin's description, that's exactly how it works.

[PT20J]

9 hours ago, aviatoreb said:

Maybe....  a trusting person would say yes - that's it.  A suspicious person would say, did the engineers really correctly consider line of sight occlusions due to a mountain between me and where I want to be?  Surely they did?  But I would want to go out and test it before I would trust it in an actual emergency.  The adjustable altitude ring is not a circle in large part because of varying altitude of the ground.  But I cannot remember ever seeing a large cut out in my range circle due to a mountain in the way - but then again I dont think Ive ever checked explicitly and mostly I fly high. ...something to do soon.

Sounds like a good excuse for a test flight. Let us know what you find out!

[1980Mooney]

1 hour ago, midlifeflyer said:

The mootness can also depend on where the failure occurs. Altitude, terrain, and airport density may be a bigger factor for success than the (significant) 450 feet of altitude  @donkaye found with a coarser prop pitch. 

But if it’s constantly doing that calculation behind the scene while you are flying, and at the moment of engine failure, it tells you that you can make a suitable airport, then you should be able to make it.  I don’t understand your point.  It is not moot. 

[midlifeflyer]

39 minutes ago, 1980Mooney said:

But if it’s constantly doing that calculation behind the scene while you are flying, and at the moment of engine failure, it tells you that you can make a suitable airport, then you should be able to make it.  I don’t understand your point.  It is not moot. 

Only that more altitude equals more options. If I am 3,000 feet right above an airport, I'm not too worried about whether the prop position (the title of the thread) is stationary or windmilling or fine pitch or course.

Maybe it's just that I don't focus on only one thing at aa time?

[donkaye]

3 hours ago, aviatoreb said:

Maybe....  a trusting person would say yes - that's it.  A suspicious person would say, did the engineers really correctly consider line of sight occlusions due to a mountain between me and where I want to be?  Surely they did?  But I would want to go out and test it before I would trust it in an actual emergency.  The adjustable altitude ring is not a circle in large part because of varying altitude of the ground.  But I cannot remember ever seeing a large cut out in my range circle due to a mountain in the way - but then again I dont think Ive ever checked explicitly and mostly I fly high. ...something to do soon.

I did when I tested it out a few days ago, as I was flying over the mountains east of Salinas where I did the testing.  I just didn't put 2 and 2 together at that time when I was just testing the system.  I'll be more careful to look next time.